This invention relates to mercury vapor fluorescent lamps and particularly to a method for maintaining the mercury pressure, and hence phosphor light output within the lamp at an optimum value by monitoring and controlling the emission of at least one of the gases contributing to the light output.
In a mercury fluorescent lamp, an electrical discharge is generated in a mixture of mercury vapor at low pressure and a fill gas, typically a rare gas such as argon, neon, Krypton, xenon or mixtures thereof. The light output from the lamp depends, among other variables, on the mercury vapor pressure inside the lamp tube. The primary radiation from the mercury is at 2537 Angstroms and arises from the transition between the lowest non-metastable excited state and the ground state. This ultraviolet radiation at 2537 Angstroms excites a phosphor which is coated inside the tube walls. The excited phosphor thereupon emits radiation at some wavelength, in the visible spectrum, characteristic of the phosphor.
It is known in the prior art that the optimum mercury pressure for maximum phosphor light output of a fluorescent lamp is approximately 7 mtorr (independent of current) which corresponds to a mercury cold spot temperature of approximately 40.degree. C. At this temperature and pressure, the light output increases monotonically with the current. At cold spot temperatures higher or lower than the optimum, phosphor, or light output falls off.
It is therefore desirable to maintain the mercury pressure at optimum at any lamp current and at any ambient temperature. Prior art techniques for accomplishing this function required a temperature-sensitive device such as a thermocouple, thermistor or thermostat to monitor the temperature of the cold spot. A feedback circuit provides closed loop control of a temperature-regulating device to maintain the optimum mercury pressure. These methods, although providing a closed loop control of the cold spot temperature, must rely on a consistent relationship of cold spot temperature to mercury density and subsequent light output which may not exist under all conditions.
The present invention is directed to a novel method for maintaining optimum mercury pressure which does not require the use of cold spot temperature measuring devices. As will be demonstrated in the succeeding descriptive portion of the specification, if lamp current is kept constant, the emission of the gas elements contained within the lamp is a function of the mercury cold spot temperature. The phrase "gas elements" is intended to include mercury in its vaporized state as well as the rare fill gases. Specifically, the fill gas emission varies inversely as the cold spot temperature and at a slope of about four times greater than that of the phosphor light output while the mercury line radiation varies directly with the cold spot. According to one aspect of the invention, the particular gas emission is continually monitored by a circuit adapted to feed back a signal to a cold spot temperature-regulating device. The circuit responds to any change in the monitored gas emission by adjusting the operation of the cold spot temperature-regulating device so as to restore the gas emission to its original value. This, in turn, restores the cold spot temperature and hence, phosphor light output to its optimum.
The advantage of this method of output control is that the phosphor light output of the lamp, which is dependent on the cold spot temperature, but which is only a unique function at optimum, can be controlled to optimum without resort to monitoring the cold spot temperature. Also, the sensitivity of particularly the fill gas emission to changes in lamp temperature permits a very accurate feedback system to be implemented as will be demonstrated below.
The present invention is therefore directed to a monitoring and control system for optimizing the phosphor output of a fluorescent lamp containing an excess of mercury at a cold spot therein, said lamp further containing a fill gas therein, said mechanism comprising:
a power supply for applying operating current to said lamp, PA1 temperature control means for varying the temperature at said cold spot, PA1 means for determining an emission level of a gaseous element contained within the lamp which corresponds to the optimum phosphor output of said lamp, PA1 means for monitoring said emission level to detect changes in said phosphor emission level, said means adapted to generate an output signal in response to a change in said emission level, and PA1 control means adapted to receive said signals from said emission monitoring means and to regulate the operation of said temperature control means, so as to maintain said cold spot temperature at an optimum level corresponding to optimum phosphor lamp output.